Ok, I have an OTTB, which as we all know are notorious for horribly thin tails. Hahaha what works for me and him (over time) is Enfusium volume shampoo/conditioner and Enfusium rejuvenation spray. Both are safe to use repeatedly and I use the rejuvenation spray every single time I brush his tail to reduce breakage. It has worked very well so far! You can buy it at any Walmart in the hair care aisle :) ENFUSIUM! It comes in a grey bottle and there are several types, choose as needed :)
Wow that's a great before and after. His tail literally looks twice as thick. I have just shampooed and massaged builder into her dock for the second time this morning. I swear her tail looks better already but that could be rose tinted glasses lol. Anyway I took a pic of her tail after a couple days TLC and also a pic of the product I'm using. I was going to wrap it but the sun has decided to come out today and there are flies about, might wrap it and at some extra wrap or string to the end so she can still swot bugs.
America has so many more products than the UK :( I'm going to see how it goes with this builder if not might have a red hot crack at flowers of sulphur or some infusium, I see you can buy it on Amazon.
I use the vet wrap method on both my mares, plus wash/condition each time I redo the wrap (probably every 6-12 weeks). After it's washed and dried I put MTG in, then wrap. Occasionally I'll throw some mane & tail moisturizing stuff in there. My pony started with her tail right at her hip before where her leg actually starts, a whispy little thing. ~6 months later it's THICK and nearly to her fetlocks.
This is late, but I do just want to add that I keep my horse's tails wrapped like this a good chunk of the year (minus some "air out" time), summer included. Mine are 100% fine with being ridden with vet wrap in. In summer when it is mosquito season - we have a LOT here, and a mare who is allergic does not help - I continue to keep her tail wrapped and as I am wrapping I tie in baling twine to act as a tail of sorts for bug swatting. Never had any issues with this.